1. Field of the Invention
This invention relates to duct systems for protecting telephone cable and other cable and conductors, and specifically to a connection adapted to facilitate the transfer of such cable from multiple bore conduits to single bore tubular ducts.
2. Description of the Prior Art
Cables and conductors, such as telephone cable, electrical cable, and current-carrying conductors, are often enclosed in protective ducts. These ducts are commonly buried and used for the installation of underground telephone cables. In many instances, a single conduit comprising multiple ducts or bores is used. Examples of early forms of these multiple duct conduits are shown in U.S. Pat. No. 1,785,403, issued to Babb; No. 1,798,205, issued to Jewell; and No. 1,835,245, issued to Shibley.
The earlier forms of these conduits were commonly formed of concrete or similar materials. In recent years, however, a foamed plastic material, such as foamed polyolefin, has been used to form multiple bore conduits. The plastic material is injection-molded to form a conduit section which is typically four feet long and has six or nine separate ducts or bores therein. These sections are then connected end-to-end to form a continuous conduit.
These multiple bore conduits are usually installed with the capability for future system expansion. Such conduits may be installed, for example, when an overhead telephone cable system is converted to an underground system, or when a new street is put in, or when a new development is started. At that time, one or more of the bores in the conduit may be used, and cables will be pulled through these bores. Typically, one or more of the bores will also remain empty to be used when necessary, such as when a new street is developed off a main artery or when a new commercial building is constructed.
A single cable or conductor is commonly encased in a hollow, tubular duct or sheath is formed of a different plastic material. These single bore tubular ducts are usually formed of polyvinyl chloride (PVC) extruded to a nominal four-inch outside diameter and typically supplied in tubular lengths of 20 or 30 feet.
When it becomes necessary to increase the capability of a telephone system, such as to provide service for a new street or building, a new cable or bundle of cables is pulled from the street or building through a single bore PVC duct which leads to the multiple bore conduit. At the point where the duct branches from the conduit, the cable or bundle of cables is then pulled through one of the unoccupied bores of conduit to an adjacent access such as a manhole or the like.
In the use of these two types of ducts, problems have arisen in providing a suitable connection between the multiple bore conduit and the single bore duct. These problems have resulted in part from the incompatible materials used in the conduit and the duct. While both the conduit and the duct are formed of plastic, they are formed of dissimilar plastic materials. The conduit is made of a lightweight, foamed plastic material, whereas the duct is formed of a heavier plastic material, such as PVC. Due to the dissimilarity of the materials, it is impossible to use chemical solvents to seal or "weld" them together, and it has generally not been possible to splice into the underground multiple bore conduit without destroying the conduit.
One solution to this problem has been to install two single bore ducts on top of the multiple bore conduit and to use these ducts for subsequent connection to branch ducts as necessary. However, the installation of the additional ducts on top of the multiple bore conduit results in ineffective use of the bores within the conduits, consumes additional lengths of cable, increases the material cost by requiring excess single bore ducts to be installed, and increases the effort and expense of installing underground conduits, since two additional ducts must be installed at the same time.